The fish finger butty is the fish finger sandwich said in a different register, and the word does real work. Butty means buttered bread, a kitchen-table name with no ceremony in it, and calling the thing a butty rather than a sandwich sets the expectation: soft sliced white, butter to the edges, breaded fish fingers laid in a row, eaten quickly and without a plate if it comes to that. The crumbed fish is the same constant it always is, a baton of white fish in a brittle orange coating. What the butty changes is not the engineering but the spirit of it. This is the sandwich at its least composed and most direct, the version made standing at the counter for someone who is hungry now.
The craft, register aside, is the same heat-and-cushion discipline the format always demands. The fingers go in straight from the pan or grill while the crumb is still crisp, because that coating is the only texture the butty has and it will not come back once it has gone soft. The bread is soft white precisely so it yields to the fingers instead of crushing them, and the butter is spread right to the edges as a thin fat barrier that keeps the slice from soaking through where the fingers sit. Sauce is a stripe, not a flood: enough to lubricate and season, not enough to steam the crust. The fingers are laid flat and side by side so the bread folds evenly over them, then pressed once so the loose row binds into something that holds for the handful of bites it is meant to last.
The variations are the same ones the sandwich always branches into, met here in the plainer register. Brown sauce or ketchup is the everyday choice where tartare would feel formal; a smear of mushy peas turns a snack into something nearer a meal; cheese melted over the fingers pushes it from cold to hot. The chip shop's battered fish in soft bread is the butty's close relative when fingers are not to hand. Each deserves its own article rather than being crowded in here.